1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a transmission power, which is intended to solve a problem with regard to a far distance and a near distance between a mobile station and a base station.
2. Description of the Related Art
Conventionally, an apparatus for and a method of controlling a transmission power is applied to, for example, an apparatus for and a method of measuring a signal to interference ratio in a radio communication system, and a method of controlling a transmission power under a code division multiple access (CDMA) communication method.
Recently, a CDMA method is noted as a transfer method used in the radio communication system. This CDMA method employs a spectrum spread that is a multiple access method in which a frequency usage efficiency is excellent. Especially, in a cellular DS/CDMA (Direct Sequence/CDMA) mobile communication, in order to maintain a necessary line quality and increase a capacity of subscribers, a technique for controlling a transmission power, which solves a so-called problem with regard to a far distance and a near distance between a mobile station and a base station, is important although it is detailed as follows.
FIG. 1 is a view showing a conceptual configuration of a radio communication system to which a typical DS/CDMA communication method is applied. This radio communication system shown in FIG. 1 is provided with a base station and a plurality of mobile stations (terminal stations). So, information, such as voice, data and the like, are transmitted and received between one base station and the plurality of mobile stations.
Actually, multiplexing through a code is carried out in the CDMA method, when the base station transmits information to the plurality of mobile stations in the radio communication system shown in FIG. 1. Thus, a signal can be transmitted to all the mobile stations at the same frequency and at the same time. Also, as one method of attaining the transmission power control in the radio communication system shown in FIG. 1, there is a closed loop transmission power control in which the base station measures a value of a signal to interference ratio (SIR) of a reception signal from each mobile station, and then controls the transmission power of the mobile station based on the value so as to maintain a predetermined transmission power.
For example, the method of measuring the signal to interference ratio is described in detail in Japanese Laid Open Patent Application (JP-A-Heisei, 10-247849) entitled xe2x80x9cAPPARATUS FOR AND METHOD OF MEASURING SIGNAL TO INTERFERENCE RATIO, AND METHOD OF CONTROLLING TRANSMISSION POWER UNDER CDMA COMMUNICATION METHODxe2x80x9d.
This apparatus for measuring a signal to interference ratio includes an interference replica signal generator, a subtracter, an interference power detector, a signal power detector and a signal to interference ratio calculator. The interference replica signal generator performs a reverse spread process on an input signal caused by a reception signal, and carries out a tentative judgment of an information symbol with regard to the input signal, and then outputs the component on which the spread process is again performed, as an interference replica signal. The subtracter subtracts the interference replica signal from the reception signal. The interference power detector detects an interference power information. The signal power detector detects a signal power information. The signal to interference ratio calculator calculates a signal to interference ratio from the interference power information and the signal power information.
Japanese Laid Open Patent Application (JP-A-Heisei, 9-139712) discloses xe2x80x9cMETHOD OF CONTROLLING TRANSMISSION POWER OF RADIO TRANSMITTERxe2x80x9d. This method is provided with the steps of: determining a standard value to indicate a quality of a signal received from a radio transmitter; determining a standard value to indicate a current traffic load in a radio cell; and instructing the radio transmitter to adjust the transmission power.
Japanese Patent No.2723094 discloses xe2x80x9cCDMA RECEIVERxe2x80x9d. This CDMA receiver is designed so as to change an operation speed of a chip synchronizer for extracting a reverse spread timing on the basis of SIR determined in accordance with a desirable signal power and an interference signal power. In this CDMA receiver, the operation speed of the chip synchronizer is reduced when the SIR is deteriorated. Thus, it is possible to protect an erroneous operation of a reception timing caused by noise.
Japanese Laid Open Patent Application (JP-A-Heisei, 11-17646) discloses xe2x80x9cMETHOD OF CONTROLLING VARIABLE RATE CDMA TRANSMISSION POWERxe2x80x9d. In this method, an Eb/Io calculator calculates an Eb/Io value of an a signal power Eb per bit to an interference power Io per Hz, for each bit rate, from a reception base band signal rs from a reception processor. A maximum value detector detects a maximum value from the Eb/Io value. A comparator compares the maximum Eb/Io value with a target Eb/Io value. A transmission power control bit, which is intended to decrease a transmission power if the maximum Eb/Io value is greater than the target Eb/Io value and increase the transmission power in the opposite case, is transmitted to a multiplexer. So, a TPC bit and a transmission data td are multiplexed. Then, the multiplexed data is modulated, and a frequency conversion is performed on it, which is transmitted from a transmitter at a power based on the TPC bit. Accordingly, without the deterioration of transfer quality, it is possible to apply to a variable rate transfer to then control the transmission power at a high accuracy.
Moreover, Japanese Laid Open Patent Application (JP-A-Heisei, 8-32514) discloses xe2x80x9cMethod of And Apparatus for Controlling Transmission Powerxe2x80x9d. In this technique, a mobile station measures an average of desirable wave reception signal powers per transmission power control period from a base station, and then detects a difference (relatively desirable wave reception signal power difference) xcex94RSSI between the average and an average in one or more transmission power control periods earlier. If the xcex94RSSI exceeds a predetermined standard power difference xcex94Pth, a transmission power control in an open loop to set a transmission power in accordance with the xcex94RSSI is carried out in order to sharply drop the transmission power of the mobile station. If the xcex94RSSI is equal to or less than the xcex94Pth, the transmission power is set in a closed loop based on the transmission power control bit. Thus, if a reception signal power at the mobile station is largely increased, the control is done so as to sharply drop the transmission power at the mobile station to thereby have no interference with other mobile stations.
However, in the conventional method of measuring the SIR, there may be a case that an SIR measurement accuracy is dropped under a fast fading environment, an interference between a plurality of stations, and a noise environment. If the transmission power is controlled in accordance with the SIR measured under above mentioned environments, this results in a problem that it has influence on the accuracy of the transmission power control.
The reason of the drop in the accuracy of the SIR measurement is as follows. That is, a value of an interference power (I) is smaller than a reception power (S). Thus, the normalization of an analog-to-digital conversion on the reception power (S) causes a drop of a accuracy of a value obtained by quantizing the interference power (I) having a relatively low level. Especially, a change of amplitude of the reception signal is large in the fading environment peculiar to the mobile communication system. This results in a problem that the accuracy of the value obtained by quantizing the interference power (I) is further dropped.
The present invention is accomplished in view of the above mentioned problems. Therefore, an object of the present invention is to provide an apparatus for and a method of controlling a transmission power, which can measure SIR having a high accuracy without any influence from the fading environment, the interference between a plurality of stations and the noise environment, by measuring an interference power in each communication channel for each sector of a base station.
In order to accomplish the object, an apparatus for controlling a transmission power according to a first aspect of the present invention includes an antenna, a plurality of processors and a total reception power calculator. The antenna receives a plurality of reception signals from a plurality of mobile stations. The plurality of processors processes said plurality of reception signals from said antenna to determine reception powers, respectively. The total reception power calculator calculates a total reception power based on said reception power outputted from each of said plurality of processors.
Each of the plurality of processors includes a reversely spreading unit, a reception power measuring unit, an SIR calculator, a transmission power control information calculator and a transmission section. The reversely spreading unit spreads said reception signal received by said antenna by using an assigned spreading code. The reception power measuring unit calculates said reception power based on the reversely spread reception signal outputted from said reversely spreading unit. The SIR calculator calculates a signal to interference ratio (SIR) of said reception signal based on said reception power calculated by said reception power measuring unit and said total reception power calculated by said total reception power calculator. The transmission power control information calculator calculates transmission power control information to make said SIR outputted from said SIR calculator equal to a predetermined SIR. The transmission section transmits said transmission power control information calculated in said transmission power control information calculator to one of said plurality of mobile stations.
In order to accomplish the object, a method for controlling a transmission power according to a second aspect of the present invention includes the steps of reversely spreading a reception signal outputted from each of a plurality of mobile stations by using an assigned spreading code, calculating a reception power based on the reversely spread reception signal, calculating a total reception power based on a plurality of the reception powers, calculating a signal to interference ratio (SIR) of the reception signal based on the reception power and the total reception power, calculating transmission power control information to approach the SIR to a predetermined SIR, and transmitting the transmission power control information to one of the plurality of mobile stations.